The present invention relates to an improvement in an electric power steering apparatus which performs a steering force assistance in a manner of setting a motor current target value of a steering force assisting motor determined on the basis of a detected value of a steering torque as a target value of automatic control, and driving the motor according to a PWM control.
There is an electric power steering apparatus which drives a steering force assisting motor according to a PWM control on the basis of a motor current target value of the auxiliary motor determined based on a detected value of a steering torque, and a detected value of a driving current of the motor. In the electric power steering apparatus, when restoring a steering wheel, a restoration current of the steering wheel is supplied to the motor so as to perform a restoration control of the steering wheel. And then, the restoration current is set to "0 (zero)" when restoring the steering wheel to the vicinity of its neutral position (the middle point of steering angle) where a vehicle goes straight.
However, even if the steering wheel is restored to the neutral position, the rotation of the steering wheel does not immediately stop at the neutral position due to an inertial force of the motor. As shown in FIG. 1, the steering wheel repeats an operation like a pendulum such that it is over the opposite side of the neutral position (0.degree.) and returns, and thereafter, converges at the neutral position. For this reason, it takes time until the steering wheel converges at the neutral position and stops, during this, a running state of the vehicle is unstable, in particular, a great influence is given to the vehicle when the vehicle is running at a high speed.
Moreover, even if the vehicle speed is the same, depending upon a steering state, there is the case where the restoration control is required, or the case where a convergence control of the steering wheel is required. For example, in the case where the rotation of the steering wheel is fast in the vicinity of the neutral position during medium vehicle speed, the convergence control is required. Also, in the case where the rotation of the steering wheel is slow in the vicinity of the neutral position during medium vehicle speed, it is the best to carry out the restoration control.
In order to solve the aforesaid problem, the present applicant has proposed an electric power steering apparatus disclosed in Japanese Patent Application No. 8-91341 (1996), together with other applicant.
In the electric power steering apparatus, a relationship between a vehicle speed, a steering angular velocity, restoration control and convergence control, is determined as shown in FIG. 2 which is a graph having an abscissa taking a vehicle speed, an ordinate taking an steering angular velocity. More specifically, the restoration control is possible when the vehicle speed is within the range of 0-30 km/h and when the vehicle speed is within the range of 30-80 km/h and the steering angular velocity is lower than 55.degree./s.
Also, the convergence control is possible when the vehicle speed is higher than 30 km/h and the steering angular velocity is higher than 60.degree./s.
In a region where the vehicle speed is within the range of 30-80 km/h and the steering angular velocity is within the range of 55.degree.-60.degree./s, the steering angular velocity has a hysteresis when carrying out a change-over of the restoration control and the convergence control. When the previous control is the restoration control, the restoration control is carried out, and when the previous control is the convergence control or usual steering assisting control (assist control), the assist control is carried out.
In the aforesaid electric power steering apparatus, however, in FIG. 2, for example, in the case where the steering angular velocity is in a state of being higher than 60.degree./s and the vehicle speed oscillates around 30 km/h, the restoration control and the convergence control are alternately changed over in short time, there has arisen a problem that hunting occurs with the steering wheel.